After running Mac OS X for some years I encountered that the “Open With…” popup menu had multiple entries of one program listed, which was kind of annoying. I tried to find solutions for this, but couldn’t find one, until I eventually ask Dr. Google the right question and found a blog post about it. Anyway, all you have to do is to start the Terminal.app and run
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/\ LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local\ -domain system -domain user
Then restart the Finder.app or restart the computer. Worked well with Mountain Lion!